Hydrophobic membranes have several different applications including membrane distillation desalination, pervaporation, dewatering solvents, gas-water vapor separation, solvent-solvent separation, acid purification, base purification, azeotrope separation, natural gas purification, pharmaceutical separation, purification of cells, yeast, proteins, bacteria, viruses, serums, and enzymes, water-gas hydrophobic barriers, sealing, venting, gas processing. Membranes are barriers that enable the separation of solutes from a solution. Membranes can separate solutes based on size, charge, or molecular diffusivity, and each type of separation is based on a different physical mechanism. Most hydrophobic membranes separate solutes based on size, a process termed size exclusion or sieving. These membranes are formed from porous materials. The size of the pores determines the size of the solutes that can be separated, but most hydrophobic membranes contain a distribution of pore sizes centered about a dominant pore size. Pore size is directly proportional to the trans-membrane flux, because larger pores generally lead to higher fluxes.
The operation of a membrane is similar to that of a sieve. The solution is flowed over the surface of the porous membrane under pressure, and solutes that are larger than the membrane pore size are prevented from passing across the barrier, while the solvent or solvents, in addition to any solutes that are smaller than the pore size, pass through the membrane. This process can be performed continuously in series to produce two solutions, one containing solutes smaller than the pore size, and one containing solutes of all sizes.
Typical hydrophobic membrane materials are polymers or ceramics. Polymeric and thin film composite membranes are often made from polysulfone, polyethersulfone, polyvinyl-alcohol, polyamide, polyacrilonitrile, polyimide, polyethylene, polypropylene, polytetrafluoroethylene, polyvinylidene fluoride, polyvinylchloride, cellulose acetate, cellulose triacetate. Ceramic membranes are typically made from zirconia oxides, titanium oxides, aluminum oxides, and silicon carbides. Various combinations of the above materials are also formed for specific separations.
Polymeric membranes are formed by dissolving the polymers in a solvent to high viscosity, then drawing, extruding, or spinning the viscous polymer solutions into their final conformation (flat sheets, thin films, hollow fibers, capillaries, tubes, fibers, or any combination of these), and then by gelling or precipitating the polymer. This final step is achieved by removing the solvent, either by evaporating the solvent through heat or by immersing the polymer-solvent solution into a non-solvent bath. Precipitating the polymer-solvent through non-solvent immersion is termed immersion precipitation, non-solvent gelation, or membrane phase inversion. The non-solvent bath is often pure water or a solution of water and a low concentration of a solvent (usually the solvent used in dissolving the membrane).
Hydrophobic membranes, a subset of membranes, have unique surface properties with little or no tendency to adsorb water. Water tends to bead on their surfaces, such as in discrete droplets, and thereby hydrophobic surfaces resist wetting. The physics of surface hydrophobicity is not precisely understood, but certain surface characteristics are known to produce hydrophobicity. Hydrophobic materials possess low surface tension values and lack active groups in their surface chemistry for formation of hydrogen bonds with water, such as carboxyl or hydroxyl groups. Greater charge density on a membrane is associated with greater membrane hydrophilicity, or an affinity for water. Most commodity polymers (excluding polytetrafluoroethylene) have surface free energies that give them only moderate hydrophobic properties. Most commodity polymers typically carry some degree of negative surface charge, and therefore have some degree of hydrophilicity. Hydrophilicity is beneficial in some circumstances, but for applications mentioned above, hydrophobicity is desired. State of the art membranes rely on chemical coatings, such as chemical modification of base layers, chemically grafted moieties, and other coatings, or inherently hydrophobic polymers such as PTFE (polytetrafluorethylene). Methods to make membrane surfaces more hydrophobic are limited in applicability and robustness. Furthermore, methods to make hydrophobic membranes from the majority of commodity polymers, which are inherently hydrophilic, are lacking.
Surface roughness has been shown to contribute to surface hydrophobicity. Uniformly structured surface roughness greatly increases the surface hydrophobicity. Uniform surface structures can be achieved through surface patterning. This has been shown to greatly increase surface hydrophobicity, but methods to do so efficiently and effectively do not currently exist. Efficient, large-scale, surface structure patterning could enable the use of common hydrophilic commodity polymers to produce hydrophobic membranes.